I was stumbling through a big pile of mail this morning and wanted to search for all messages whose subject did not contain a certain keyword. It turns out you can do some decent searching in Mail.
Straight from the Help:
When you search entire messages, you can refine your search by using logical symbols to represent “and” (&), “or” ( | ), “not” ( ! ), and parentheses. For example:
“cat & dog” finds messages containing both “cat” and “dog”
“cat | dog” finds messages containing either “cat” or “dog”
“cat ! dog” finds messages containing “cat” but not “dog”
“cat & (dog | newt)” finds messages containing both “cat” and “dog” and messages containing both “cat” and “newt”